Geminga
From Wikipedia, the free encyclopedia
| Observation data Equinox J2000.0 |
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|---|---|
| Constellation | Gemini |
| Right ascension | 06h 33m 54.15s |
| Declination | +17° 46′ 12.9″ |
| Apparent magnitude (V) | ? |
| Distance | 552 ly (169 pc) |
| Spectral type | Pulsar |
| Other designations | |
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SN 437, PSR B0633+17
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Geminga, is a neutron star approximately 552 light-years away in the constellation Gemini. Its name is a contraction of "Gemini gamma-ray source", and coincidentally means "it's not there" in the Italian Milanese dialect (Bignami et al. 1983).
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[edit] Pulsar
The nature of Geminga was quite unknown for 20 years after its discovery by the SAS-2 satellite. Finally, in March 1991 the ROSAT satellite detected a periodicity of 0.237 s in soft x-ray emission. Thus, it is supposed that Geminga is a sort of neutron star: the decaying core of a behemoth star that went supernova about 300,000 years ago.[1]
This nearby explosion may be responsible for the low density of the interstellar medium in the immediate vicinity of the Solar System. This low-density area is known as the Local Bubble.
[edit] Discovery and identification
Geminga was the first example of an unidentified gamma-ray source, a source which could not be associated with any objects known at other wavelength. It was first detected as a significant excess of gamma-rays over the expected background of diffuse Galactic emission, by the SAS-2 satellite (Fictel et al. 1975) and subsequently by the COS-B satellite. The SAS-2 group reported a pulsation in the gamma-ray signal, with period approximately 59 s, although the limited number of detected gamma-rays (121 over a period of four months) led them to conclude that the pulsation was not statistically compelling. Due to the limited angular resolution of the instrument (approximately 2.5° at 100MeV) and the small number of gamma-rays detected, the exact location of the source was uncertain, constrained only to be within a relatively large "error region". At the time of detection, four weak radio sources were known within this region, two supernova remnants bordered it and a known satellite galaxy to the Milky Way lay nearby. None of these known sources were convincing associations to the gamma-ray source, and the SAS-2 team suggested that an undiscovered radio-pulsar was the most likely progenitor (Thompson et al. 1977).
Despite the investment of a significant amount of observation time, the source remained unidentified through the COS-B era; their data did, however, rule out the claimed 59 s pulsation. Many claims were made about the source during this time, but its nature remained a mystery until the identification of a candidate source by the Einstein x-ray satellite, 1E 0630+178 (Bignami et al. 1983). The characteristics of the x-ray source were unique: large x-ray to optical luminosity, no radio emission detected by the sensitive VLA instrument, point-like emission in the Einstein imager and an estimated distance of approximately 100 pc, placing it within the Galaxy. An association between the gamma-ray and x-ray sources was not conclusively made until the ROSAT x-ray imager detected a 237 ms pulsation (Halpern&Holt 1992), which was also seen in gamma-rays by the EGRET instrument (Bertsch et al. 1992) and retrospectively in the COS-B and SAS-2 data (Bignami&Caraveo 1992, Mattox et al. 1992).
Geminga is the first example of a radio-quiet pulsar, and serves as an illustration of the difficulty of associating gamma-ray emission with objects known at other wavelengths: either no credible object is detected in the error region of the gamma-ray source, or a number are present and some characteristic of the gamma-ray source, such as periodicity or variability, must be identified in one of the prospective candidates (or vice-versa as in the case of Geminga).
[edit] Possible planetary system
In 1997, Mattox et al. claimed to have discovered a planet orbiting Geminga by gamma-ray timing of Geminga. This hypothesized planet, Geminga b, was thought to orbit about 3.3 AU from Geminga in a 5.1 year orbit. With a mass of 1.7 earths, Geminga b would be a terrestrial planet. However, this discovery is now doubtful because recent analysis of the data indicates that the detected timing changes were due to signal noise, not a planet.
[edit] External links
- Spaceflight Now: 'Cannonball pulsar' seen flying across space
- ESA: Hipparcos pinpoints an amazing gamma-ray clock
- Extrasolar Visions: Geminga + Geminga b
[edit] References
- G. F. Bignami et al. An identification for ’Geminga’ (2CG 195+04) 1E 0630+178 - A unique object in the error box of the high-energy gamma-ray source. ApJ, 72:L9–L13, September 1983.
- C. E. Fichtel et al. High-energy gamma-ray results from the second small astronomy satellite. ApJ, 198:163–182, May 1975.
- D. J. Thompson et al. Final SAS-2 gamma-ray results on sources in the galactic anticenter region. ApJ, 213:252–262, April 1977.
- J. P. Halpern and S. S. Holt. Discovery of soft X-ray pulsations from the gamma-ray source Geminga. Nature, 357:222–224, May 1992.
- D. L. Bertsch et al. Pulsed high-energy gamma-radiation from Geminga (1E0630 + 178). Nature, 357:306, May 1992.
- G. F. Bignami and P. A. Caraveo. Geminga - New Period Old Gamma-Rays. Nature, 357:287, May 1992.
- J. R. Mattox et al. SAS 2 observation of pulsed high-energy gamma radiation from Geminga. ApJ, 401:L23–L26, December 1992.
